Solar thermal power plants operating on conventional oil based technology are known from practice. Such known conventional oil based solar thermal power plants comprise a solar collection system having one or more tube radiation absorbers and a plurality of trough collectors, such as single axis parabolic reflectors or Fresnel collectors configured to receive and reflect incoming solar radiation onto a radiation absorber, which is formed as a tube. A thermal fluid, constituting a heat transfer medium, which is typically a liquid such as oil, flows within the radiation absorber tube. The thermal energy transported by the thermal fluid is used to generate steam which in turn is expanded into a turbine, in order to generate electricity in a conventional way, by coupling the axle of the turbine to a generator. Then, the steam is condensed and recycled back to a heat exchanger system. The cycle to transfer steam into electricity may be more efficient at high steam temperatures. However, the oil based thermal fluid may dissociate or otherwise break down or become less effective at transferring heat if said heat transfer media operate at very high temperatures. Due to this restricting behaviour, conventional plants generating solar thermal energy are typically limited to operate at about 400° C. At such temperature the overall efficiency of the cycle to transfer steam into electricity may be limited to less than 32%.
An alternative known solar thermal power plant may be based on using molten salts as heat transfer fluid (HTF). Such a solar thermal power plant always needs to operate at a temperature higher than the solidification temperature of the molten salts (around 240° C. for the Na/K nitrates mixture) and can be used to an operating temperature of approximately 550° C. Auxiliary heaters or boilers may additionally be used. However, the known solar thermal power plants may still have efficiency problems.
Therefore, it is an object of the present invention to provide a method for modifying a solar thermal power plant operating on conventional oil based technology such that the overall efficiency of the cycle to transfer steam into electricity of said solar thermal power plant may be improved.